45-150 - Cheap and small with decent quality
45-175 - X range with power zoom for video a little sharper
45-200 - Older heavier and larger with more reach, as sharp as the other till 150 and a little faster.

Why there are 3 of them, probably there is a market need for such long tele and Panasonic decided to have a few offering for different market needs.
They are all good lenses any of then you chose will do its work.

Law of diminishing returns, as with all gear. You pay premium price for quality improvements most people won't really notice in prints or web. But you probably will. I know I do for the higher quality zooms (my experience is with Canon more than MFT, but the same sort of thing applies).

What would be great would be a 'Canon-like' line of F4 zooms. High grade glass, constant aperture, expensive but not ridiculously so.